1. Field of the Invention
The present invention relates to a dielectric filter. More specifically, the present invention relates to a dielectric filter in which a plurality of coaxial resonators are formed in a dielectric block and a printed circuit board is disposed at a side of an open end of the dielectric block.
2. Description of the Prior Art
The same assignee of the present invention has proposed such a kind of dielectric filter in, for example, Japanese Patent Application No. 254452/1986 (Japanese Patent Laid Open No. 108801/1988 laid open on May 13, 1988).
Such a dielectric filter is constituted by a plurality of coaxial resonators formed on a dielectric block, as shown in FIG. 6. The dielectric filter having such construction is well known, and therefore, more detail description thereof is omitted here.
In this proposed dielectric filter, in order to easily adjust a filter characteristic by trimming or to form an attenuation pole in the filter characteristic, at a side of an open end of the dielectric block 1, a printed circuit board 2 to which predetermined coaxial resonators are connected is disposed, and the dielectric block 1 and the printed circuit board 2 are covered by a metallic case 3 to which an outer conductor (ground electrode) is connected.
In the dielectric filter as shown in FIG. 6, since the printed circuit board 2 is fixed closely to metallic case 3, a large stray capacity is formed between a hot pattern (not shown) of the printed circuit board 2 and the metallic case 3. Since such a stray capacity is greatly changed in accordance with a state or degree of contact of the printed circuit board 2 and the metallic case 3, change or fluctuation of resonant frequencies of the coaxial resonators occur, and therefore, a whole filter characteristic is changed. It takes a long time to adjust the filter characteristic so as to avoid influence of the stray capacity upon the filter characteristic. Thus, there was a problem in the above described dielectric filter that mass-productivity is not so good.